Organic Compounds 1

Organic Compound Properties In general, not very soluble in water Uncharged or weakly charged Can exist as dissolved, solid, or gaseous phases Organic matter in water is composed of an almost infinite variety of compounds – Most dissolved organic matter in groundwater are humic acids – Very resistant to further biodegradation 2

Measuring Organic Compounds in Groundwater Dissolved organic carbon (DOC) (water passed through 0.45 μm filter) DOC in groundwater typically low, ≤ 2 mg/L Swamps and other wetlands can have much higher DOC values, ~60 mg/L 3

Organic Compound Nomenclature All organics have carbon skeletons with functional groups attached Aliphatics: straight or branched chains Aromatics: ring structure – Multi-rings = polyaromatics (PNAs or PAHs) – Heterocyclic: ring structure with atoms other than C in skeleton 4

Organic Compound Functional Groups Besides H, skeleton can have other functional groups attached to it which effect compound properties – Sites of reactivity or function Impart important properties to organic compounds – Charge, polarity (sharing of electrons, affects solubility), acidity, adsorption, chelation Alcohol (or hydroxyl): OH group – Most common – H dissociates, weak acid 5

Ethanol 6

Organic Compound Functional Groups Carboxyl: R – COOH – Weak acids; e.g., acetic acid (CH 3 COOH) – Strong H + donors/acceptors, increase solubility because of charge – Easily degraded 7

Carboxyls 8 Formic Acid Acetic Acid

Organic Compound Functional Groups Halogens (Cl -, Br -, I -, F - ) – Can be naturally occurring, but contaminants associated with anthropogenic production such as pesticides (DDT), solvents (TCE), refrigerants (CFCs, PCBs) – Halogens strongly bonded to C atoms, stable compounds in the environment – Low solubility because weak H-bonding with H 2 O – Trihalomethanes form in chlorinated drinking water 9

Halogens 10 DDT TCE Generally the more halogen atoms, the more resistant to degradation

Organic Compound Functional Groups Amino: NH 2 – Better proton acceptors, weak acids – From H bonds with H 2 O, increase solubility – Amino acids: building blocks of life 11

Amino 12

Organic complexes Organic compounds can bond with ions Especially important with respect to metals – Can increase metal solubility and mobility e.g., natural waters commonly have Fe concentrations several orders of magnitude greater than the equilibrium solubility of iron hydroxide Fe may form dissolved complexes with naturally occurring organic substances Ligands = ion or molecule (usually organic) that binds to a metal atom – Have a negative charge 13

Chelation A special type of aqueous complex (strong bonds) Most ligands: single bond site (unidentate) Chelation: multidenate (2 or more bonds with cation/metal) – Multiple bonds decrease entropy, increase bond stability Can increase mobility of metals significantly 14

Chelation Natural chelating agents – Humic and fulvic acids – Citric acid Anthropogenic chelating agents – Polyphosphates: water softeners that complex with Ca 2+ to inhibit precipitation of CaCO 3 – NTA and EDTA: cleaning compounds, detergents, metal plating baths Very stable in environment (EDTA also food preservative) and have been implicated in 60 Co transport at Oak Ridge National Lab 15

EDTA (C 10 H 16 N 2 O 8 ) 16

DOC in Natural Environments Soils: O and A horizons are major source of DOC to soil water and groundwater – DOC decreases with depth in soil profile Decomposition Adsorption Precipitation as a solid – Organic acids can control pH of soil and therefore mineral weathering – Al/Fe can complex with organics, increasing transport to lower horizons 17

DOC in Natural Environments Groundwater: – Usually < 2 mg/L since most is removed in soil zone – Can be higher under certain conditions, e.g., buried paleosols (ancient soils) – Can be important in increasing transport of metals and radioactive elements Implicated in solubility of As in sand and gravel aquifers in Illinois Rivers – Varies by climate, season, vegetation, and discharge Low discharge, groundwater main source of water High discharge, increasing % of soil water 18

Organic Pollutants Large number of synthetic organics, many of which find their way into the environment Relevant properties: – Solubility – Adsorption – Density – Liquid/gas partitioning – Biodegradability 19

Organic Pollutants 3 main groups which cause most problems (due to abundance and toxicity) – Aromatic hydrocarbons: fuels, BTEX (benzene, toluene, ethylbenzene, xylene) – Chlorinated hydrocarbons: solvents, pesticides – PAHs: low solubility, but carcinogenic 20

Solubility of organics (Considering only synthetic organics of known chemical composition) In general, hydrophobic – Repelled by water – Low solubility Even though only slightly soluble in water, their equilibrium solubility can be 1000 – 1 million times greater than the regulatory MCL 21

Solubility of organics Usual measure of hydrophobicity (literally, “fear of water”) is octanol-water partitioning coefficient – Octanol (CH 3 (CH 2 ) 7 OH) is a liquid (alcohol) – Octanol and water are immiscible fluids i.e., they don’t mix (like oil-water) Use of octanol is arbitrary, but it is a non-polar organic liquid (water is polar) Polar solutes dissolve in polar solvents – e.g., alcoholic beverages are aqueous solutions of ethanol Non-polar solutes dissolve better in non-polar solvents – e.g., hydrocarbons such as oil and grease that easily mix with each other, while being incompatible with water 22

Octanol-water partitioning coefficient Determine using batch tests – Mix octanol, water, and organic of interest, and measure concentration in both phases – K ow = C octanol / C H2O K ow = octanol-water partitioning coefficient C octanol = equilibrium concentration of compound in octanol C H2O = equilibrium concentration of compound in water – K ow came from biosciences field, to determine behavior of organic compounds in living organisms 23

Octanol-water partitioning coefficient K ow correlated to water solubility As K ow increases, hydrophobicity increases and solubility decreases In general, K ow good 1 st approximation for solubility, and also indicator of adsorption and bioaccumulation 24

Some K ow values 25 Compoundlog K ow Ethanol Ethyl acetate Pentanol1.39 Nitrobenzene1.84 Benzene2.14 Chlorobenzene2.80 Biphenyl3.96 Pentachlorobenzene4.99 Decreasing solubility

Adsorption of Organics Also known as partitioning (between aqueous and solid phases) Since most organics are hydrophobic, they tend to adsorb onto aquifer solids Organics are attracted to the solid organic matter – Unlike charged surfaces, there is not a theoretical limit to the amount of adsorption that can occur – What often occurs is multilayer adsorption 26

Adsorption of Organics Perform batch experiments to determine partitioning coefficient between the dissolved and adsorbed amounts of a given organic – Plots of data have linear and nonlinear parts – Linear partitioning coefficient may be appropriate at low concentrations – At higher concentrations, Freundlich isotherms used C* = K f C N K f = Freundlich partitioning coefficient N = fitting parameter 27

Different Isotherms 28

Freundlich Isotherm C* = K f C N Make log-log plot of batch test data – Take the log of both the aqueous (C) and adsorbed (C*) concentrations – log C* = N log C + log K f N (slope) commonly 0.9 – 1.4 log K f = y-intercept 29

Freundlich Isotherm 30 log C* log K f

Adsorption of Organics Can calculate K d from K ow – As K ow increases, solubility decreases, K d increases – Organic adsorption is a function of the amount of organic matter in the soil or aquifer material (f oc ) As f oc increases, adsorbed mass increases, K d increases f oc usually ranges 1 – 5 % in soil When f oc < 1%, organic adsorption ≈ inorganic adsorption – K d = K oc f oc K oc = organic carbon partitioning coefficient 31

Adsorption of Organics K d = K OC f OC – K OC = the ratio of the mass of a chemical that is adsorbed in the soil per unit mass of organic carbon in the soil per the equilibrium chemical concentration in solution – Normalized – K OC values useful in predicting the mobility of organic soil contaminants; High K OC values = less mobile organic compounds Low K OC values = more mobile organic compounds 32

Adsorption of Organics K oc is calculated from K ow values – Empirical equations developed based on type of organic – e.g., for aromatic/PAHs, log K oc = log K ow – Steps for converting K ow to K d – Look up K ow ; Calculate K oc ; Measure f oc ; Calculate K d – For many organic chemicals, K d strongly correlated to their aqueous solubilities Main advantages: simple (look up), and f oc easily measured 33